Alright here's a little guide:
1.) Find a guitar pick or a credit card.
2.) Holding the metal ipod bottom firmly in one hand, take your other hand and push the white top over, exposing a gap between the top and bottom parts of the ipod.
3.) Stick the pick or the corner of the credit card into the gap, try to get it to go in about 3/4 of a centimeter.
4.) Now carefully slide the pick/card around the seem of the ipod, this will undo the clips that hold it together, and the white top and metal bottom should come apart.
5.) Once you have separated the top and bottom, flip the top portion upward so that it lies on its face, be careful of the copper connector tape, do not break or detach it.
6.) Lift up the harddrive (big metal thing with blue rubber around it, and detach it from the connector which is under a sort of foam pad. Set the harddrive aside.
7.) The battery is the little black thing under the hard drive, lift it up and work the wiring out from around the motherboardish thing. Then using your fingernails unclip the little battery connecter near the top of the little white box thing.
8.) Dispose of the dead battery (there is some responsible way to do this, but i don't know what it is)
9.) Now take your new battery and work the wires around and under the mother board thing.
10.) Then plug the little battery connection into the spot where you unplugged the dead battery.
11.) Stick the battery in the space and jam the wires in around it so that it all fits.
12.) Plug the hard drive back in and snap the top back on your ipod making sure that the lock switch is in the same position on both halves of the ipod.
Good luck.